The objective of this proposal is to assess the structural complexity of circulation human B cells and to assign in vivo functional roles to the subjects identified. These objectives are pursued by in vivo immunization of normal individuals, singly or repetitively, with tetanus toxoid followed by in vitro fractionation and culture of B cells. The different B-cell populations are identified by cell surface markers, size, reactivity to different mitogens and temporal appearance in the circulation after immunization. Studies in the coming years will in particular examine the mechanism(s) of antigen-induced inhibition of pokeweed mitogen-stimulated antibody synthesis. Additional studies will assess changes in immunoglobulin isotype expression following introduction of an in vivo antigen-specific refractory state by multiple immunizations.